The present invention relates to a press roll, and in particular relates to a press roll for removing water which is provided in the press portion of a paper machine, and a paper machine having the press roll.
Conventionally, a roll made of natural granite which is a so-called "stone roll", has been widely used as a press roll. However, such stone rolls are expensive due to the exhaustion of natural good-quality granite. Furthermore, the stone roll is difficult to produce and is not sufficiently strong. Therefore, recently, a roll having a plastic layer formed on the surface of a steel roll, and a spray-coated roll having a sprayed coating 2 on the outer surface of a steel roll 1 (as shown in FIG. 4) have been researched and developed so as to replace the stone roll.
However, the plastic-steel roll does not have sufficiently good performance to permit smooth release of wet paper and does not have high surface hardness; the plastic-steel roll therefore easily causes abrasion or thrust by the doctor blade which is a blade for removing bits of extraneous stock such as paper dregs adhering on the roll. Therefore, use of the plastic-steel roll cannot necessarily yield satisfactory results.
The sprayed roll has several other problems in that the sprayed coating 2 thereof is easily soiled and cleaning thereof is difficult. Furthermore, the sprayed roll does not exhibit sufficiently good. Performance to permit smooth release of wet paper, and the sprayed coating 2 is often damaged by freezing.
Oxide ceramics having high hardness, e.g., alumina, titania, zirconia or the like, have been employed for the sprayed coating 2 of the sprayed roll which has been conventionally used, taking into consideration the performance thereof so as to permit smooth release of wet paper and so as not to cause abrasion by the doctor blade. However, the sprayed coating 2 comprising such a ceramic has often voids of several .mu.m to several 10's of .mu.m among particles of the ceramics, the voids comprising several % to several 10's of % of the total area ratio. The voids often are interconnected in three dimensions and form deep pinholes or conduits which pass through the sprayed coating 2 from the outer surface thereof to the surface of the metal base steel roll 1.
Rosin (collophonium) or aluminium sulfate as an inhibitor of ink bleeding, i.e., a sizing agent, when printing, and a kind of loading material, e.g. , clay, talc, gypsum, barium sulfate, magnesium carbonate or the like, for improving paper quality, i.e., opacity, surface smoothness, flexibility, glossiness, density, uniformity, stretch prevention or the like, are added to the pulp liquid. Some of the particles of these additives are small enough to easily enter into the voids of sprayed coating 2. If these small particles penetrate into the voids, it is impossible to remove them because the diameter of the voids is very small. These small particles are gradually deposited in the voids, so that the deposits finally come into direct contact with the wet paper. Consequently, the performance with regard to the smooth release of wet paper deteriorates and causes problems such as breakage of the paper. There is a possibility of the failure of the sprayed coating or the peeling off thereof from the metal base by volume expansion of the pulp liquid if the pulp liquid in the voids freezes.